The present invention relates to a process for cleaning quartz and silicon surfaces which are contaminated with inorganic substances. In particular, the invention related to the cleaning of such surfaces for application which require a clean and smooth surface. The surface may be any surface of a quartz or silicon semiconductor device, or the surface may be the quartz surface (usually inner) of equipment used in the manufacture of semiconductor devices. For example, the invention is useful in cleaning silicon wafers or disks.
The present invention also relates to a process for cleaning quartzware used in the manufacture of semiconductor devices. The types of quartzware used in such a manufacture include quartz diffusion tubes, quartz dummy wafers, quartz injector tubes, quartz paddles, quartz cantilevers, quartz pull rods, quartz wafer boats, quartz thermocouple sheaths, quartz bell jars, and epitaxial reactor vessels. In the interest of maintaining purity, it is common to manufacture semiconductors in vessels made of quartz. Quartz diffusion tubes and quartz epitaxial reactors are used in the semiconductor industry to carry out chemical vapor deposition. During a chemical vapor deposition, for example, silicon wafers are enclosed within the quartz diffusion tube, and then vapors of, for example, mixtures of silicon tetrachloride and hydrogen are introduced into the tube. These vapors act on the silicon wafer to form the desired silica deposit on the wafers. These materials may also deposit on the reaction tube and subsequently flake off and contaminate the film surface. These processes also result in the deposition of silicon (polysilicon) as well as other metals and metal compounds on the inner surface of the tube. Before the quartz tube can be reused, these contaminants must be removed from the inner surface of the tube. Conventionally, this has been accomplished by immersing the tube in a vessel containing a reagent, usually a mixture of nitric acid and hydrofluroic acid, and then rinsing the tube thoroughly with deionized water. It also has been suggested to clean the surface by heating the vessel under vacuum or in hydrogen.
Use of the above-mentioned conventional process of cleaning with one or more inorganic acids has not been entirely satisfactory because the acids, in addition to removing the deposits on the surface, will also etch the quartz surface and convert the previously smooth surface to a rather rough surface which has a greater tendency to become contaminated in subsequent use and which results in a reduction in yield of acceptable wafers. Thus, the process of cleaning a quartz surface with acids may be followed by a polishing step. Each time the quartzware is cleaned with acid, the acid etches away an additional amount of the quartz from the quartzware, increases the surface area of the quartzware, and reduces the remaining life of the part. If a diffusion tube is cleaned by immersion in strong acid, etching of the exterior surface of the tube also results thereby further unnecessarily shortening the life of the tube.
In addition, acids such as nitric acid and hydrofluoric acid require special handling procedures because the acids are hazardous since they are highly corrosive and toxic, and can cause severe and painful chemical burns if brought into contact with human skin. Moreover, the reaction of nitric acid with the silicon produces a toxic nitrous oxide gas which requires that the cleaning operation be conducted under special equipment such as a ventilated hood, and there is still the possibility of exposure to the toxic gas.
Procedures have been described in patents and in various publications for cleaning the surfaces, and particularly the inner surfaces, of quartzware. In the article entitled "Quartzware Cleaning Technology" in Solid State Technology, Vol. 31, No. 1, January 1988, pp. 79-80, a nine-step cleaning cycle is described which includes several rinsing steps, a degreasing step utilizing heated choline (trimethyl-2-hydroxyethyl ammonium hydroxide), a concentrated acid etching step which is reported to remove virtually all deposits including polysilicons from the quartzware, and a polishing step using a dilute acid. The degreasing step is reported to remove organic contaminants such as finger oils, greases, particles and organic compounds without any degrading residue. The degreasing step is accomplished by placing the quartzware in an enclosed exhausted compartment which locks prior to a timed dispense of heated dilute choline.
U.S. Pat. No. 4,239,661 describes a surface-treating agent for removing organic and inorganic contaminants deposited on the surface of intermediate semiconductor products. The surface-treating agent which is also useful for eliminating those portions of a positive working photoresist film coated on the surface of the intermediate semiconductor product comprises an aqueous solution containing one or more of trialkyl(hydroxyalkyl)ammonium hydroxides. Examples of such hydroxides identified in the patent include trimethyl(2-hydroxyethyl)ammonium hydroxide, also referred to as "cholin", triethyl(2-hydroxyethyl)ammonium hydroxide, tripropyl(2-hydroxyethyl)ammonium hydroxide, etc. The patentees report that where the surface portion of intermediate products obtained in the manufacturing of a semiconductor device is washed with the surface-treating agent, the semiconductor product is immersed for 5 to 20 minutes and the agent maintained in a 70.degree.-90.degree. C., and the product is preferably subjected to ultrasonic cleaning. Silicon wafers are used as an example of the intermediate semiconductor product which can be treated with the agents described in the patent.
The use of acids and acid mixtures for cleaning deposited matter such as silicon and other contaminants from the interior of a quartz diffusion tube and other quartzware is described in, for example, U.S. Pat. No. 4,294,271, and various publications such as White, "Clean Surface Technology", Symp. on Freq. Control, 27th Ann. Proc. Pap., Cherry Hill, N.J. Jun. 12, 1975, pp. 79-88 and J. Bardina, "Methods for Surface Particle Removal: Comparative Study", Particulate Science and Technology 6:121-131, 1988. In the publication by White, aqueous systems comprising hydrogen peroxide; a mixture of hydrogen fluoride and nitric acid; and a mixture of hydrofluoric acid with ammonium fluoride are described. In the Bardina publication, various etching solutions are described on page 128, and these include a mixture of water, hydrogen peroxide and ammonium hydroxide.
U.S. Pat. No. 3,673,099 describes the procedure for stripping cured resins such as silicones or polyvinyl cinnamate from substrates by exposure to a mixture of N-methyl-2-pyrrolidone and a strong base such as an alkyl or substituted-alkyl ammonium hydroxide. Compatible solvents may also be included such as ethylene glycol monoethyl ether.
U.S. Pat. No. 4,089,704 describes a procedure for removing RTV silicon rubber encapsulants from electronic circuits. The encapsulant is broken down molecularly and removed by a solution of tetramethylammonium hydroxide which is diluted with an alcohol such as 2-propanol. This stripping solution is reported to cause minimal swelling of the encapsulant and leaves no significant residue which cannot be removed by standard rinsing.